Field of the Invention
The invention relates to an amplifier output stage containing a final stage and a preliminary stage connected to the final stage and having a multiplicity of adjustable current stages for setting a quiescent current in the final stage. A control device is connected to the preliminary stage and has a control current from which control voltages are generated and regulated in accordance with the multiplicity of adjustable current sources of the preliminary stage. Each of the control voltages controls precisely one of the adjustable current sources in the preliminary stage.
Published European Patent Application EP-0115949 A1 describes a buffer circuit having a high impedance. The buffer circuit has a main buffer circuit section formed from a pair of complementary input transistors whose base terminals are connected to one another, and also an additional circuit section having the same circuit structure as the main buffer circuit section. A feedback signal generated by the additional circuit section is fed to the additional circuit section and to the main buffer circuit section in order that an output signal of the additional circuit section assumes a predetermined value.
In amplifiers which have to supply high currents for driving large loads, a complementary emitter/source follower in push-pull class AB operation is usually used as the final stage (see the reference by U. Tietze, Ch. Schenck, titled xe2x80x9cHalbleiterschaltungstechnikxe2x80x9d [Semiconductor Circuitry], 9th edition, pp. 516 to 518 and pp. 521 to 523). In this operating mode, a parallel-path current, referred to as a quiescent current, continuously flows through the complementary final-stage transistors in a non-driven state of the amplifier. Power is consumed as a result of this quiescent current, which results in that the final-stage transistors are heated. It proves to be particularly problematic to set this temperature-dependent quiescent current to a low and constant value over a large temperature range, in order to minimize the current consumption in the final stage and to avoid distortions.
IEEE Journal of Solid-State Circuits, Vol. 27, No. 4, pages 539-544, April 1992, titled xe2x80x9cA CMOS Line Driver with 80-dB Linearity for ISDN Applicationsxe2x80x9d, by H. Knorramabadi, discloses a line driver amplifier for an integrated services digital network (ISDN) U interface, in which a so-called error amplifier which drives the final stage is used. A disadvantage of such a configuration is that the error amplifier lies directly in the signal path of the amplifier and influences the bandwidth of the amplifier. Moreover, the quiescent current through the final-stage transistors depends on an offset voltage of the error amplifiers.
IEEE Journal of Solid-State Circuits, Vol. 32, No. 32, pages 169-176, February 1997, titled xe2x80x9cA Rail-to-Rail, Constant Gain, Buffered Op-Amp for Real Time Video Applicationsxe2x80x9d, by L. Moldovan and H. H. Li, and IEEE Journal of Solid-State Circuits, Vol. 29, No. 1, pages 63-65, January 1994, titled xe2x80x9cDigital-Compatible High-Performance Operational Amplifier with Rail-to-Rail Input and Output Rangesxe2x80x9d, by W. -CH. S. Wu, W. J. Helms, J. A. Kuhn, and B. E. Byrkett describe amplifiers in which the quiescent current in the final stage is set by an input stage and is thus directly dependent on the input signals and current or voltage fluctuations of the input stage.
In the reference titled xe2x80x9cIntegrierte Schaltungenxe2x80x9d [Integrated Circuits], by A. Schlachetzki, and W. V. Munch, Teubner 1978, a complementary emitter follower with quiescent current setting is illustrated in FIG. 4.8 on page 144, in the case of which the quiescent current to be set is mirrored into the final-stage transistors T1 and T2 via transistors T3 and T4 connected as diodes. However, the quiescent current is not freely adjustable in this configuration.
It is accordingly an object of the invention to provide an amplifier output stage which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which has better adjustability and regulation of the quiescent current in the final stage and in which the quiescent current regulation and setting lie outside the signal path of the amplifier.
With the foregoing and other objects in view there is provided, in accordance with the invention, an amplifier output stage. The amplifier output stage contains a final stage, a preliminary stage connected to the final stage and has a multiplicity of adjustable current sources for setting a quiescent current in the final stage, and a control device connected to the preliminary stage and having a control current from which control voltages are generated and regulated in accordance with the multiplicity of adjustable current sources of the preliminary stage. Each of the control voltages control precisely one of the multiplicity of adjustable current sources in the preliminary stage, and the control current in the control device is proportional to the quiescent current in the final stage.
One embodiment of the amplifier output stage contains the preliminary stage, the final stage and the control device. In this case, the quiescent current through the transistors of the final stage is set by the preliminary stage, which is connected to the control device. The preliminary stage has controllable current sources, by which the voltage at the control terminals of the final-stage transistors is set. This voltage defines the quiescent current through the final-stage transistors. In the control device, a control current is set, which is proportional to the quiescent current flowing through the final-stage transistors. From this, control voltages are generated for setting the current sources in the preliminary stage, which in turn set the quiescent current in the final stage. It is advantageous that the control device lies outside the signal path of the amplifier output stage and, consequently, does not limit the bandwidth of the amplifier output stage. In the control device, moreover, the control current and thus the quiescent current can be set to an arbitrary and exact value.
In a preferred embodiment of the invention, the preliminary stage of the amplifier output stage has two emitter or source follower circuits that the final stage decouples from an amplifier input stage which drives the amplifier output stage. This advantageously reduces the load on the amplifier input stage since the preliminary-stage transistors driven by the amplifier input stage represent a lower capacitive load than the final-stage transistors for the input stages. Each emitter or source follower circuit has a controllable current source that sets the quiescent current through the transistors of the final stage. In this embodiment, it is advantageous that only one transistor lies in the signal path of the amplifier output stage, the limiting frequency of which transistor is so high that the bandwidth of the amplifier is not restricted.
A preferred embodiment of the control device has a first and second emitter or source follower circuit, which each simulate the emitter or source follower circuits of the preliminary stage in the non-driven state. Each emitter or source follower circuit has a controllable current source. The inputs of the two emitter or source follower circuits are connected to a voltage corresponding to the output voltage of the amplifier output stage in the quiescent state. It is advantageous that similar operating conditions apply to the emitter or source follower circuits of the preliminary stage and of the control device.
In a preferred embodiment, the final stage has a first and second transistor which are operated in push-pull class AB operation. Advantages include the lower harmonic distortion factor compared with a final stage in class A operation and the lower current consumption compared with a final stage in class B operation.
In a preferred embodiment, the control device furthermore has a first and second transistor and an adjustable current source. In this case, the two transistors simulate the two transistors of the final stage and the current source feeds a control current into both transistors. It is advantageous that the control current fed in is adjustable and proportional to the quiescent current in the final stage.
In a preferred embodiment, the control device has a first and a second mirror transistor, the first mirror transistor mirroring the current through the first transistor and the second mirror transistor mirroring the current through the second transistor. The voltage at the control terminals of the mirror transistors is advantageously used for generating the control voltages.
The control device particularly preferably has a first and a second control transistor, the control transistors transferring the input voltage at the inputs of the emitter follower or source follower circuits to the first and second transistor. Consequently, the simulated final stage has a quiescent output potential which advantageously corresponds to the input voltage at the inputs of the emitter follower or source follower circuits and the quiescent output potential can be set by way of the input voltage of the emitter follower or source follower circuits.
In a particularly preferred embodiment of the control device, a first and second operational amplifier regulates the current sources contained in the emitter or source follower circuits in such a way that the voltage at the output of the operational amplifier can be used for setting the current sources in the preliminary stage. It is advantageous here that the voltage is dependent only as a result of the offset voltage of a few mV of the operational amplifiers and on the ratio of a few transistors of the current mirror and the final stage. This regulation of the control voltage has the major advantage that it is very accurate and disturbing influences (OP offset voltage, transistor ratios) have a negligible influence on the control voltage.
In a particularly preferred embodiment of the invention, the amplifier output stage is embodied using CMOS technology. Advantages of this embodiment include the capability of integration with further circuits using CMOS technology, and also the widespread use of CMOS and the relatively small area requirement compared with other technologies.
In a preferred embodiment, the current mirrors of the control device of the amplifier output stage have transistor ratios which conduct the current impressed into the two current mirrors almost completely through in each case one of the two transistors of the current mirrors. As a result, a potential which, applied to the control terminals of the transistors of the final stage, produces a defined quiescent current through the transistors of the final stage is advantageously established at the control terminal common to both current mirror transistors.
Through a preferred dimensioning of the ratios of in each case a transistor of the current mirror and a transistor of the final stage, the quiescent current through the transistor of the final stage can be set very precisely. A ratio of 20 is particularly preferred here, which ratio can be established very simply through the dimensioning of the transistors.
In accordance with an added feature of the invention, the control current in the control device is a fraction of the quiescent current flowing through the first transistor and the second transistor of the final stage. The fraction is determined by a channel width/length ratio of the second transistor of the control device, by a channel width/length ratio of the second transistor of the final stage, by a channel width/length ratio of the first transistor of the control device, and by a channel width/length ratio of the first transistor of the final stage.
The integration of the amplifier output stage on an integrated circuit is a preferred embodiment. A great advantage here is the good adjustability of the ratios of the transistor geometries.
In a particularly preferred embodiment, the emitter or source follower circuits of the preliminary stage and of the control device are placed locally next to one another on an integrated circuit. This advantageously reduces the influence of process fluctuations on the quiescent current through the transistors of the final stage. Moreover, temperature differences between the individual circuit sections of the amplifier output stage have a negligible influence on the quiescent current.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an amplifier output stage, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.